What defines a compound in chemistry?

In the context of chemistry, a compound is specifically defined as a substance formed when two or more different elements chemically bond together. This definition emphasizes the necessity of a chemical bond, which can include ionic bonds, covalent bonds, or other types of interactions that result in the formation of a new substance with distinct properties from its constituent elements.

When elements combine chemically, they alter their individual characteristics and create a compound with unique properties that are not found in the separate elements. For instance, when hydrogen and oxygen combine to form water, the properties of water differ significantly from those of hydrogen gas and oxygen gas.

The other options do not accurately capture the definition of a compound. A pure substance that cannot be broken down chemically refers to an element, not a compound. The description of a mixture pertains to compounds that have been physically combined without any chemical bonds, rather than those that are chemically bonded. Lastly, a reaction between acids and bases that produces salt is a specific type of chemical reaction and does not represent the broader definition of what constitutes a compound.